Natural gas fired engines are large, having up to 16 cylinders, and often generating between 500-3000 HP. The engines are typically used in the Oil and Gas industry to compress natural gas at well heads and along pipelines. Due to the nature of this application, the engines often run continuously near full load conditions, shutting down only for maintenance such as for oil changes. This condition of running continuously near full load places severe demands on the lubricant. Indeed, since the lubricant is subjected to a high temperature environment, the life of the lubricant is often limited by oil oxidation processes. Additionally, since natural gas fired engines run with high emissions of oxides of nitrogen (NO.sub.x), the lubricant life may also be limited by oil nitration processes. Therefore, it is desirable for gas engine oils to have long life through enhanced resistance to oil oxidation and nitration.
The combustion of diesel fuel often results in a small amount of incomplete combustion (e.g., exhaust particulates). The incombustibles provide a small but critical degree of lubrication to the exhaust valve/seat interface, thereby ensuring the durability of both cylinder heads and valves. The combustion of natural gas is often very complete, with virtually no incombustible materials. Therefore, the durability of the cylinder head and valve is controlled by the properties of the lubricant and its consumption rate. For this reason, gas engine oils are classified according to their ash content, since it is the lubricant ash which acts as a solid lubricant to protect the valve/seat interface. The oil industry has accepted guidelines which classify gas engine oils according to their ash level. The classifications are:
______________________________________ Ash Designation Ash Level (wt %, ASTM D874) ______________________________________ Ashless Ash &lt; 0.1% Low Ash 0.1 &lt; Ash &lt; 0.6 Medium Ash 0.6 &lt; Ash &lt; 1.5 High Ash Ash &gt; 1.5 ______________________________________
The ash level of the lubricant is often determined by its formulation components, with metal-containing detergents (e.g., barium, calcium) and metallic-containing antiwear additives contributing to the ash level of the lubricant. For correct engine operation, gas engine manufacturers define lubricant ash requirements as part of the lubricant specifications. For example, manufacturers of 2-cycle engines often require the gas engine oil to be Ashless in order to minimize the extent of harmful deposits which form on the piston and combustion chamber area. Manufacturers of 4-cycle engines often require the gas engine oils to be Low, Medium or High Ash to provide the correct balance of engine cleanliness, and durability of the cylinder head and valves. Running the engine with too low an ash level will likely result in shortened life for the valves or cylinder head. Running the engine with too high an ash level will likely cause excessive deposits in the combustion chamber and upper piston area.
Gas engine oil of enhanced life as evidenced by an increase in the resistance of the oil to oxidation, nitration and deposit formation is the subject of U.S. Pat. No. 5,726,133. The gas engine oil of that patent is a low ash gas engine oil comprising a major amount of a base oil of lubricating viscosity and a minor amount of an additive mixture comprising a mixture of detergents comprising at least one low Total Base Number (TBN) alkali or alkaline earth metal salt having a TBN of about 250 and less and a second alkali or alkaline earth metal salt having a TBN lower than the aforesaid component. The TBN of this second alkali or alkaline earth metal salt will typically be about half or less that of the aforesaid component.
The fully formulated gas engine oil of U.S. Pat. No. 5,726,133 can also typically contain other standard additives known to those skilled in the art, including dispersants (about 0.5 to 8 vol %), phenolic or aminic anti oxidants (about 0.05 to 1.5 vol %), metal deactivators such as triazoles, alkyl substituted dimercaptothiadiazoles (about 0.01 to 0.2 vol %), anti wear additives such as metal di thiophosphates, metal dithiocarbamates, metal xanthates or tricresyl-phosphates (about 0.05 to 1.5 vol %), pour point depressants such as poly (meth) acrylates or alkyl aromatic polymers (about 0.05-0.6 vol %), anti foamants such as silicone antifoaming agents (about 0.005 to 0.15 vol %), and viscosity index improvers, such as olefin copolymers, polymethacrylates, styrene-diene block copolmyers, and star copolymers (up to about 15 vol %, preferably up to about 10 vol %).